Wow, I've never seen that flying window animation before. I assume you do not have a multi-monitor setup so there's no reason for the Firefox window to be off-screen to the right.
Can you tell from the Task Bar tile/button whether Firefox is the active application? If so, could you try the trick described in this thread and see whether it works for you: [https://support.mozilla.org/en-US/questions/941896 When I go to open firefox it shows up on my tool bar but not on my screen where is it?]
Any luck?

Make sure the window is "Restored" (not Maximized) by pressing Alt+spacebar then R

Move the window by pressing Alt+spacebar then m then pressing/holding the left arrow key until you get the window title bar enough into view that you can click and drag it, then press Enter

Could you try "moving" the window to the left? Here's how:
With a Firefox window active (judging from the task bar or Alt+Tab):
* Make sure the window is "Restored" (not Maximized) by pressing Alt+spacebar then R
* Move the window by pressing Alt+spacebar then m then pressing/holding the left arrow key until you get the window title bar enough into view that you can click and drag it, then press Enter